[Python-3000] str.format vs. string.Formatter exceptions

[Ron Adam]

Eric Smith wrote:
> Ron Adam points out some differences in which exceptions are thrown by 
> str.format and string.Formatter.  For example, on a missing positional 
> argument:
> 
>  >>> "{0}".format()
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> ValueError: Not enough positional arguments in format string
> 
>  >>> Formatter().format("{0}")
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
>    File "/shared/src/python/py3k/Lib/string.py", line 201, in format
>      return self.vformat(format_string, args, kwargs)
>    File "/shared/src/python/py3k/Lib/string.py", line 220, in vformat
>      obj, arg_used = self.get_field(field_name, args, kwargs)
>    File "/shared/src/python/py3k/Lib/string.py", line 278, in get_field
>      obj = self.get_value(first, args, kwargs)
>    File "/shared/src/python/py3k/Lib/string.py", line 235, in get_value
>      return args[key]
> IndexError: tuple index out of range
> 
> The PEP says: In general, exceptions generated by the formatter code 
> itself are of the "ValueError" variety -- there is an error in the 
> actual "value" of the format string.


The PEP also says the following in regards to this...

+----------------
     Implementation note: The implementation of this proposal is
     not required to enforce the rule about a name being a valid
     Python identifier.  Instead, it will rely on the getattr function
     of the underlying object to throw an exception if the identifier
     is not legal.  The str.format() function will have a minimalist
     parser which only attempts to figure out when it is "done" with an
     identifier (by finding a '.' or a ']', or '}', etc.).
+----------------

If these return ValueErrors,  as I think it has been suggested in the 
earlier messages, then this will need to be updated as well.

_RON



> I can easily change string.Formatter to make this a ValueError, and I 
> think that's probably the right thing to do.  For example, if the string 
> comes from a translation module, then there might be an extra parameter 
> added by mistake, in which case ValueError seems right to me.
> 
> But I'd like to hear if anyone else thinks this should be an IndexError, 
> or maybe they both should be some other exception.
> 
> Similarly "{x}".format()' currently raises ValueError, but 
> 'Formatter().format("{x}")' raises KeyError.
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